The present invention relates to power systems and their backup battery power systems and more particularly to a transient free reconnection of disconnected loads after a mains power failure.
Power systems for telecommunication applications of today usually use the full float operation concept consisting of busbars to which rectifiers, batteries and loads are connected in parallel. A typical operating DC voltage level is between xe2x88x9240.5 and xe2x88x9260 volts to comply with a standard for operation of xe2x88x9248 volts telecommunications systems. The batteries, which normally constitute lead accumulators, are sensitive to deep discharge and must therefore be protected from such an excessive discharge. A common way of protection is to disconnect the batteries from the busbars by connection/reconnection devices (heavy duty switches) in the case of a too long lasting mains power failure. The switching member then forms a battery contactor device, which is normally referred to as a LVBD (Low Voltage Battery Disconnect) which is illustrated in FIG. 1.
In new applications it is economically beneficial to first disconnect low priority loads during mains failures. This means that a disconnecting device for each load, a LVLD (Low Voltage Load Disconnect) has to be incorporated into the power system to prevent a deep discharge of the batteries in case of such a long lasting mains power failure. A typical basic system according to the state of the art is illustrated in FIG. 2.
The existing solutions have the following disadvantages:
LVBD (FIG. 1)
When several batteries are connected to the power supplying busbars the maximum system size is given by the size of the disconnection/reconnection device since the last disconnection device normally will have to disconnect the total load current. Furthermore priority/non-priority load disconnection is not possible.
LVLD (FIG. 2)
Transients on system voltage (voltage sags produced by large inrush currents to the capacitors in the load) at reconnection of individual non-priority load to live busbars may disturb the operation of the priority loads. Lesser load power supply availability compared to the LVBD principle (failure of disconnection/reconnection device causes interruption of the power supply to the load). Stress on the disconnecting/reconnecting device since the device has to handle the total non-priority load current.
UK Patent Application GB-A-2 281 458 discloses an apparatus for managing power supply to a telecommunications base station. This disclosure is representative of the state of the art.
A further document U.S. Pat. No. 5,260,605 discloses a polarity switch which changes the direction of DC to various loads in response to a sensed loss of AC and nonessential loads are shed in response to a sensed change in DC polarity at the various loads.
Finally a Japanese document JP-04-071338 discloses a solution to increase backup time by a battery without increasing the capacity of the battery by reducing a load current in response to a decrease in output voltage even if interruption of a power source for a communication is continued.
Typically according to state of the art power systems are described wherein loads are selectively disconnected upon a power failure while maintaining a reduced DC operation by means of backup batteries and the entire load is then reconnected for full operation immediately when AC power is established again. Furthermore there are also solutions available utilizing one common rectifier similar to FIG. 2, but distributing DC from the battery to loads of different priority by means of a diode circuitry. However when operating with larger current loads the voltage drop over such diodes may often be of the order 1-1,5 volts resulting in a non-desirable high power loss besides an unwanted heavy heating of the diodes in the circuitry. Furthermore such a system can not in a simple way be expanded as the current of the priority load or loads will pass a distribution diode, which therefore will limit the priority system size.
Therefore there is still a demand for a DC power supply system, which is able to connect and disconnect loads, particularly at high DC current consumption, according to a predefined priority in connection to a mains AC power loss, without stressing the contacts of the switching means due to transients at disconnect or strong inrush currents at connect during such a switching while trying to maintain a certain level of backup battery charge.
The present invention discloses a full float system with Transient Free Multiple Load Disconnection Reconnection (TFMLDR), which is based on a concept of always connecting loads and dedicated rectifiers together, and then connect such groups via power switching members to a respective busbar of a pair of busbars. The batteries will be connected directly to this busbar preferably via fuses. The disconnection/reconnection of individual groups to a busbar can then be controlled such that no transients are produced in the system voltage. Such a system is advantageous for reconnection of a load after a disconnection due to a mains power failure. The loads present a substantial input capacitance assisting in stabilizing DC power supply to the load and this load is directly connected to a rectifier delivering voltage controlled power during normal mains operation. After a disconnection of a load from the busbar and the battery by the power switching member, a reconnection will be done in the following order. The rectifier delivers current controlled power, which charges the input capacitance of the load up to a voltage level close to the voltage of the battery. Then reconnection with limited inrush current will be achieved thereby obtaining a practically transient free reconnection to the battery.